There are lots of reasons for disagreeing. People’s personal experiences shape the paradigm through which they filter their thoughts and opinions. For example, if a murder occurs in a family, that event may well affect the members and friends of the victim’s opinions on crime, law enforcement, capital punishment, gun control, etc.
People’s adoption of a certain set of religious beliefs or code of living can impact their opinions on matters outside the credo.
Finally, the human brain can hold 11 contradictory trains of thought. Further, as evidenced on this site, not all people accepts facts as facts.
Most of the time, if respectful, disagreement is a good method of airing all sides of an issue. I think it might be boring if we were all Stepford thought clones.